Methods for producing chemicals such as organic acids, e.g. acetic acid and alcohols, e.g. ethanol from microbial fermentation of gaseous substrates comprising carbon monoxide and hydrogen in media containing suitable nutrients and trace minerals using certain microorganism, such as those from the genus Clostridium, have been demonstrated. For example, U.S. Pat. No. 5,173,429 to Gaddy et al. discloses Clostridium ljungdahlii ATCC No. 49587, an anaerobic microorganism that produces ethanol and acetate from synthesis gas. U.S. Pat. No. 5,807,722 to Gaddy et al. discloses a method and apparatus for converting waste gases into useful products such as organic acids and alcohols using anaerobic bacteria, such as Clostridium ljungdahlii ATCC No. 55380. U.S. Pat. No. 6,136,577 to Gaddy et al. discloses a method and apparatus for converting waste gases into useful products such as organic acids and alcohols (particularly ethanol) using anaerobic bacteria, such as Clostridium ljungdahlii ATCC Nos. 55988 and 55989.
US Patent Application No. 20070275447 discloses a clostridium bacterial species (Clostridium carboxidivorans, ATCC BAA-624, “P7”) that is capable of synthesizing, from waste gases, products which are useful as biofuel. U.S. Pat. No. 7,704,723 discloses a clostridium bacterial species (Clostridium ragsdalei, ATCC BAA-622, “P11”) that is capable of synthesizing, from waste gases, products which are useful as biofuel.
WO 2007/117157 discloses use of Clostridium autoethanogenum (Accession No. DSM 10061, DSMZ, Germany) for the production of ethanol by anaerobic fermentation of substrates containing carbon monoxide. WO 2009/064200 discloses another bacteria (Clostridium autoethanogenum, Accession No. DSM 19630, DSMZ, Germany) for the production of ethanol by anaerobic fermentation of substrates containing carbon monoxide.
As described in the art, rate of production of chemicals such as alcohol depend on density of microbial cells (“cell density”) in the fermentation medium. Adequately high cell density in the bioreactor is required in order to attain and maintain a high rate of production of chemicals.
U.S. Pat. No. 6,136,577 to Gaddy discloses a process of ethanol production in a fermentation process wherein cell-recycle is used to increase cell density.
U.S. Pat. No. 7,285,402 to Gaddy et al. discloses an anaerobic microbial fermentation process for the production of alcohol wherein a method of increasing cell density is presented during start up using a stock culture wherein there was excess H2 present.
Start-up using a batch inoculum from stock culture ensures a healthy inoculum free from contaminants, but is not always successful as an inoculation procedure because of the rather low cell density employed, especially if the method parameters such as gas rate and agitation rate are pushed upward too rapidly just after inoculation.
Currently, there is a need in the art for improved methods to increase cell density in microbial fermentation of a gaseous substrate. The present disclosure provides a method to increase cell density at a faster rate for microbial fermentation methods of a gaseous substrate.